When to Switch from GPT-4 to GPT-4o Mini (Cost vs Quality)
Our document tagging feature was costing $1,200/month on GPT-4.1. We ran a quick test: same prompts, GPT-4o mini. Quality was nearly identical for our use case. We switched. Next month: $180. An 85% cut. We celebrated. Then we tried the same thing with our support chatbot. Bad idea. When to switch from GPT-4 to GPT-4o mini depends entirely on your use case. Here's how we figured it out.
GPT-4 vs GPT-4o mini: the cost gap
GPT-4o mini costs roughly $0.15/1M input and $0.60/1M output. GPT-4.1 costs $2/1M input and $8/1M output. That's a 10–13x difference. For high-volume features, the GPT-4o mini cost savings are massive. The question is whether the quality holds up. For some workloads it does. For others it doesn't.
Where the switch to GPT-4o mini is a no-brainer
Simple classification, extraction, summarisation of straightforward text, basic Q&A — the quality gap is often tiny. We moved document tagging, email categorisation, and simple intent detection to mini. Thousands of requests per day. The savings added up fast. When to use GPT-4o mini: any high-volume, well-defined task where the output doesn't need nuance. Switch GPT-4 to cheaper model for those first.
Where we stayed on GPT-4
Our support chatbot needed nuance. One wrong answer — "yes, you can cancel and get a refund" when the policy said otherwise — cost us more than a month of token savings. We kept the main chat on GPT-4.1. The cost per conversation was higher, but the quality was non-negotiable. Code generation? Same. Complex reasoning? Same. GPT-4o mini cost savings aren't worth it when one bad output costs more than the savings.
How we decided: A/B test
We routed 10% of traffic to mini for a week. Compared support tickets, user feedback, conversion. For document tagging, the numbers held. For support chat, they didn't. One feature switched, one stayed. Your mileage will vary — test on your use case before you switch GPT-4 to GPT-4o mini across the board.
To estimate savings before you switch, use our model comparison tool. Once you know cost per feature, PerUnit gives you the breakdown by customer and tier.